Technical Field
The present description relates to techniques for receiving Global
Navigation Satellite System (GNSS) signals according to multiple standards, comprising at least a first GNSS signal at a first frequency corresponding to a “Globalnaya Navigatsionnaya Sputnikovaya Sistema” (GLONASS) center frequency and a second GNSS signal at a second frequency corresponding to a “B{hacek over (e)}id{hacek over (o)}u Wèixīng D{hacek over (a)}oháng Xìt{hacek over (o)}ng” (BeiDou) center frequency, said receiver comprising a mixer for mixing said plurality of
GNSS signals with a local signal at a local frequency to generate a corresponding plurality of mixed signals and a low intermediate frequency section.
Various embodiments may apply, e.g., in GNSS receivers able to process al the GNSS signals in the L1 band (1550-1610 MHz).
Description of the Related Art
The Global Navigation Satellite System (GNSS) performs better when signals from many satellites are received concurrently: the maximum number of available satellites is reached when a receiver is compatible with all the existent GNSSs; which are the global positioning system (GPS) belonging to USA, the European Galileo, the Russian GLONASS, and the Chinese BeiDou2, in particular BeiDou2 B1I. For each GNSS service frequency band, among the other parameters, the RF receiver chain must endeavor also:                a good out-of-band attenuation to cut off noise and to mitigate strong interfering tones, which may saturate the following ADC circuits;        a good aliasing attenuation for a correct operation of the ADC;        a good image rejection to avoid the degradation of signal-to-noise (SNR) ratio due to the overlapping of image band noise or, in worst case, of parasitic signals present in the same image band.        
Normally, typical GNSS receivers are based on the Low Intermediate Frequency (Low-IF) architecture, demanding to the IF section, mainly to the IF Filter, the coverage of these three parameters.
To this regard in FIG. 1A it is shown a single chain receiver 10, including a receiving antenna 11, which receives a plurality of GNSS signals including GPS signal SGP, a Galileo signal SGA, a GLONASS signal SGL, a BeiDou2 signal SBE, and delivers them to a radio frequency (RF) receiving section 12, including an amplifier 13 and a mixer 14, which mixes the incoming signals with a local oscillator signal SLO operating at a local frequency fLO. Downstream the mixer 14 originates a plurality of corresponding mixed signals, i.e., a mixed GPS signal S′GP, a mixed Galileo signal S′GA, a mixed GLONASS signal S′GL, a mixed BeiDou2 signal S′BE, whose respective bands as the result of the sum and the difference with the local frequency fLO, as known, are shifted in a main signal, i.e., the signal which is usually taken in account, and an image signal. To this regard, in FIG. 1B it is shown a diagram representing in the frequency domain the GNSS signals, specifically the band of the GPS signal SGP with a center frequency fGP at 1575 MHz, the band of the Galileo signal SGA with a center frequency fGA also at 1575 MHz, the band of the GLONASS signal SGL with a center frequency fGL at 1601 MHz, the band of the BeiDou2 signal SBE with a center frequency fBE at 1561 MHz. Also it is shown the local oscillator signal LO frequency fLO. Also there are shown, in the right portion of FIG. 1B the main mixed signals S′GP, S′GA, S′GL, S′BE, downstream the mixer 14. The image signal, not shown, lies in the frequencies lower than zero because of the value of the local frequency fLO. A low-IF section 15 receives the mixed GNSS signals S′GP, S′GA, S′GL, S′BE generated by the mixer 14 and filters them through an IF filter 16, which has filter shape adapted to select the mixed GNSS signals S′GP, S′GA, S′GL, S′BE as received signals RGP, RGA, RGL, RBE, after amplification by an AGC (Automatic Gain Control) circuit 17 comprised in the Low IF section 15. In particular, in the example shown in FIG. 1B the filter function F is a band-pass with a bandwidth B, between 0 and 46 MHz. The received GNSS signals RGP, RGA, RGL, RBE are then fed to an ADC (Analog to Digital Converter) circuit 18, which output is then supplied to a digital section (not shown) for base band processing.
Indeed, although not shown in FIG. 1 for simplicity, the mixer 14 is of the type using two instances in quadrature, with 0° phase and 90° phase, of the local signal SLO. Thus, as indicated by the two lines outputted by the mixer 14 in FIG. 1, the mixed signals generated by the mixer 14 include an in-phase signal and a quadrature signal. In the same way, filter 16 is indeed a complex filter, in order to operate on such quadrature signals. This approach is in any case known, for instance as quadrature down-conversion.
On the basis of what has just been discussed with reference to FIG. 1, since the GLONASS L1 band is about 8 MHz around 1601 MHz and the BeiDou2 L1 band is about 4 MHz around 1561 MHz, the difference between the two band centers is 40 MHz. The GPS and Galileo frequencies remain included between such previous two services. In the case of a receiver with a single chain for all services, such frequency difference results very stringent because the IF section applies a filter with at least 46 MHz of bandwidth B, this implying a high current consumption and a high silicon area occupation. Depending on the technology, this result may not even result be feasible.
Thus with a single chain, the concurrent reception may be difficult especially for GLONASS and BeiDou2, whose bands show the widest gap.
Other methods have been presented, for instance methods based on a unique zero-IF chain that delivers the output signal in complex format, are not able to perform the image rejection. Choosing a local oscillator operating at a frequency fLO˜1582 MHz, approximately in the middle of the four bands, the GLONASS service lies in the real part of the spectrum, with an upper frequency fmax˜23 MHz; GPS, Galileo and BeiDou2 are received as image frequencies, with an upper frequency fmax˜23 MHz, that is at the same frequencies of GLONASS, but with different phase. Thus, a base band digital processing is then necessary to correctly receive all four services.
Therefore this approach has the following drawbacks:                the 23 MHz low pass band filter may still be hard to obtain with conventional low cost silicon technology. Up to now, the known implementations have a low pass filter with 18 MHz band, which is not enough to receive concurrently all services. A high performance-high cost silicon technology may be necessary to accomplish such task;        more operations are left to the base band digital processing.        
The RF receivers that are able to manage the GLONASS and BeiDou2 signals, and then all GNSS services, concurrently may typically use a multiple chain, that means the use of three branches with three mixers, three dedicated IF filter, three AGC circuits, and three ADC circuits. Although all these blocks are easily feasible also using a silicon technology with ordinary performances, since just about 6 MHz and 10 MHz are the upper frequencies of the filters, and the ADCs must run at lower sample frequencies than the previously described zero-IF solutions, the results are quite expensive in terms of hardware resources and of power consumption.
The subject matter discussed in the Background section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in the Background section. Along these lines, any recognition of problems in the prior art discussed in Background section or associated with such subject matter should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion of any subject matter in the Background section should be treated as part of the inventor's approach to the particular problem, which in and of itself may also be inventive.